Final Four Hours

The crew spent the next few hours of their approach by making extensive checks of their electrical power, environmental and propulsion systems as well as their caution and warning system. If a final mid-course correction was required, this was the time to make it, and if a correction was necessary, some additional onboard navigation sightings were taken in case communications were lost in the final hours.

Before reaching the waters of the Pacific Ocean, a coordinated sequence of events had to occur in the correct order, including a number of explosive devices. These would separate the command and service modules before re-entry and operate the various subsystems of the ELS during the final drop through the denser layers of the atmosphere. This included the jettisoning of the forward heatshield to uncover the parachutes, and their subsequent deployment. These complex events were under the control of the sequential events control system (SECS), whose circuits had to be armed by the crew, but whose status could only be verified by mission control via

Diagram showing the orientation of the entry REFSMMAT.

telemetry. At this early stage, a check was made of these circuits by the crew temporarily arming them and asking the flight controllers to inspect them.

With an hour and a half remaining, the guidance platform was given its final realignment in accordance with the previously uploaded entry REFSMMAT. Additionally, and as a backup, the same alignment was passed to the spacecraft's secondary attitude reference, which consisted of the strapped-down gyroscopes and their associated electronics system, the gyro display couplers (GDCs). This just required a press of the 'GDC Align' button, and although this backup system was more prone to drift, if all went well the spacecraft would be in the water before it became an issue.

Seating for the crew put the command module pilot in the left seat as he had received most training for the re-entry procedures. None of the Apollo crews wore their pressure suits for re-entry after the Apollo 7 crew rebelled, citing their head colds as a reason not to have to wear the bulky garments. The CMP manoeuvred the spacecraft to an attitude that put the heatshield forward with the crew heads-down and looking back. It wasn't their final entry attitude; a further pitch-up would be required for that - a manoeuvre they would execute a few minutes before reaching entry interface. But in this attitude, they could make checks of their attitude control and trajectory, and it was a starting point for the jettisoning of the service module.

The first of these checks required that the sextant be aimed at an angle given in the PAD, with the expectation that, if their attitude was where it should be, a specified star would be visible in the instrument's narrow field of view. With that check made, the spacecraft's optics had performed their last task, and were swung to a shaft angle of 90 degrees and powered down. Earlier flights also included an additional check of attitude using the COAS - an optical sight, similar to a gun sight, mounted in one of the rendezvous windows. Again, with the sight mounted at specified angles, a named star was expected to be visible through the instrument.

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